It is preferred to include synthetic or natural calcium compounds in bone implants to increase bone conductivity, i.e., growth of bone cells. Examples of such calcium compounds include calcium phosphate, calcium sulphate, deproteinated bone (inorganic bone), calcium composites, and calcium carbonate.
Various methods have been developed for preparing bone implants containing calcium compounds. For example, one can coat a metal matrix with calcium phosphate by plasma spraying treatment, a high-cost process. See, e.g., U.S. Pat. No. 6,846,853. As another example, one can prepare a collagen-based implant impregnated with calcium phosphate as follows: dispersing collagen fiber in a phosphate-containing solution, adding a calcium-containing solution at a pH value of higher than 8 to precipitate calcium phosphate, and freeze-drying the collagen-calcium phosphate mixture to form a sponge-like or membrane-like material. See, e.g., U.S. Pat. Nos. 6,764,517; 6,187,047; 5,455,231; 6,417,166; and 6,300,315. The calcium-impregnated bone implants thus obtained can be too brittle to manipulate.